**The Dumbbell Nebula**: Messier 27, a planetary nebula in Vulpecula

Night of 17 October 2026, San Francisco Bay Area

**Equipment**

* Telescope: Celestron 8SE Schmidt-Cassegrain: 203 mm aperture, 2,082.4 mm focal length by plate solve of this stack, f/10.3
* Camera: Sony a6000, stock, at prime focus: APS-C 23.5 x 15.6 mm, 6000 x 4000, 3.9 micron pixels, RAW
* Mount: Sky-Watcher EQ6-R, unguided, not dithered on this target. Set down about 10 degrees off the pole; aligned from four plate-solved photos (under 1 arcminute), both axes driven from that model
* Computer: Raspberry Pi 3 running Observatory, in Elixir

**Acquisition**

* Lights: 66 × 15 s at ISO 3200 (16.5 minutes)
* Frames: 70 taken, 66 kept
* When: The evening of 17 October 2026, over 36 minutes. About 60 degrees up
* Sky: Clear and still. A Moon a little under half lit, low in the southwest early on and soon behind the garage
* Darks: None taken tonight. Hot pixels found from the run itself, about 20,000 to 21,500 per colour plane, and repaired by the 3 x 3 median of their colour plane
* Flats: No flat frames tonight. The hybrid flat of 13 October: its large scale from that night's open sky, its dust from the 13 October twilight frames. Picked over two others on this run's own sky: it leaves 0.57 percent in green after a plane, against 0.80 and 1.98 for the others
* Calibration: Dust that moved since 13 October, measured on this run's own sky, seen in the odd and the even frames alone, and divided out: 11 shadows, 0.73 percent of the sensor. Among them a new crescent 12.7 percent deep, which the 13 October flat does not have, left at 1.7 percent
* Field: 15.5 x 12.9 arcmin
* Sampling: 0.777 arcsec per pixel

**Processing**

* Registration: Rotation and shift on the stars; the field drifted 3.7 arcmin across the sensor and turned 0.05 degrees. Red and blue star images sat 0.3 sensor px off green (the air's prism): measured and taken out in the one resampling
* Stacking: RAW colour planes (no demosaic), hot pixels by 3 x 3 median, registered on stars with rotation, proper coaddition (Zackay and Ofek: each frame weighted by its own measured blur and noise). One sky constant per colour per frame; in the picture only, a sky plane per colour, about 3 DN across it. Noise 7.5 to 7.8 times lower than one frame (8.1 expected from the weights). The old sigma-clipped mean was made beside it as a control
* Finish: Star colours from Gaia, the camera's colour matrix. Arcsinh stretch; black at the 50th percentile, S-curve 0.15, saturation x 1.2; the dark sky smoothed (bilateral 4 px) and set near black, by choice: 4.0 percent of the picture is exactly black
* Software: Capture and mount: Observatory (Elixir on a Raspberry Pi 3). Stacking: numpy, OpenCV, rawpy. Plate solving: astrometry.net. Stacked from the camera's RAW frames. Nothing generated

Full write-up: https://bradgessler.github.io/observatory-blog/observations/2026-10-17/dumbbell-nebula.html

All the photography notes: https://bradgessler.github.io/observatory-blog/observations/2026-10-17/dumbbell-nebula.html#photography